Tailoring Electrode–Electrolyte Interface Using an Electron-Deficient Borate-Based Additive in MgTFSI2-MgCl2/DME Electrolyte for Rechargeable Magnesium Batteries

Abstract

Rechargeable magnesium metal batteries need an electrolyte that forms a stable and ionically conductive solid electrolyte interphase (SEI) on the anodes. Here, we used molecular dynamic simulation, density functional theory calculation, and X-ray photoelectron spectroscopy analysis to investigate the solvation structures and SEI compositions in electrolytes consisting of dual-salts, magnesium bis(trifluoromethanesulfonyl)imide (MgTFSI2), and MgCl2, with different additives in 1,2-dimethoxyethane (DME) solvent. We found that the formed [Mg3(μ-Cl)4(DME)mTFSI2] (m = 3, 5) inner-shell solvation clusters in MgTFSI2-MgCl2/DME electrolyte could easily decompose and form a MgO- and MgF2-rich SEI. Such electron-rich inorganic species in the SEI, especially MgF2, turned out to be detrimental for Mg plating/stripping. To reduce the MgF2 and MgO contents in SEI, we introduce an electron-deficient tri(2,2,2-trifluoroethyl) borate (TFEB) additive in the electrolyte. Mg//Mg cells using the MgTFSI2-MgCl2/DME-TFEB electrolyte could cycle stably for over 400 h with a small polarization voltage of ~150 mV. Even with the presence of 800 ppm H2O, the electrolyte with TFEB additive could still preserve its good electrochemical performance. The optimized electrolyte also enabled stable cycling and high-rate capability for Mg//Mo6S8 and Mg//CuS full cells, showing great potential for future applications.